A number of solutions have been proposed for long-term storage and disposal of various waste streams. The options for disposing of a particular type of waste will depend in part on the nature of the waste. Safely and cost-effectively disposing of hazardous wastes, for example, presents a difficult challenge. Such hazardous waste streams may include one, two, or more of aqueous liquids, heterogeneous debris, inorganic sludges, heavy metals, organic liquids, contaminated soils, and radioactive byproducts of nuclear power generation or weapons manufacture. (As used herein, the term “hazardous waste” may include nuclear materials that may not be classified as “hazardous waste” under pertinent state, federal, or local laws or regulations.) High-level radioactive waste also presents significant processing difficulties.
One of the approaches proposed for long-term stabilization and storage of hazardous wastes, particularly radioactive wastes, is vitrification. Unfortunately, vitrification requires very high temperature processing. For example, U.S. Pat. No. 6,258,994 suggests vitrification of waste, including radioactive waste, at about 1,050-1,250° C. and states that conventional vitrification processes take place at, e.g., 1,400° C. Heating the waste to such high temperatures is quite costly. Many hazardous waste streams include hazardous materials that volatilize at “light-off” temperatures well below 1,000° C. Some hazardous components of radioactive waste streams, for example, have light-off temperatures as low as 200° C., with mercury chloride volatilizing at about 200-225° C. As a consequence, vitrifying a waste including mercury chloride or other low light-off temperature materials generates a secondary hazardous waste stream requiring further processing.
Others have proposed immobilizing or stabilizing hazardous wastes in ceramics that can be formed at lower temperatures. International Publication No. WO 92/15536 (the entirety of which is incorporated herein by reference), for example, suggests immobilizing hazardous waste in hydrated cement. A variety of chemically bonded phosphate ceramic (CBPC) products have been used to stabilize hazardous waste. For example, U.S. Pat. Nos. 5,645,518 and 5,846,894 and U.S. Patent Application Publication 2003/0092554 (the entirety of each of which is incorporated herein by reference) suggest various CBPC compositions useful for low-temperature waste processing. Conventional CBPCs suggested for waste processing are typically hydrous ceramics such as magnesium potassium phosphate hexahydrate (MgKPO4.6H2O) or newberryite (MgHPO4.3H2O).
Hydrated cements and CBPCs have proven to be quite useful in handling a variety of waste streams. Unfortunately, conventional cements and CBPCs have proven somewhat problematic for stabilizing radioactive wastes, particularly high-activity radioactive wastes. Radioactive wastes typically radiate γ rays and α, β, and n particles, which can decompose the bound water in hydrous cements and CBPCs in a process referred to as radiolysis to generate hydrogen gas. This hydrogen gas pressurizes storage containers or other waste forms, which can cause the containers or waste forms to fracture and admit intrusion of moisture from air, groundwater, or other elements. Under some circumstances, water can reflect nuclear radiation, increasing the chance that highly active radioactive wastes could “go critical” if the waste loading is not kept artificially low.
The significant volume and weight of the final waste form are also shortcomings of waste storage employing CBPCs and hydrated cement compositions. If the waste stream is dry or is a liquid waste with relatively low water content, additional water must be added to form the ceramic matrix. This increases both the volume and the weight of the final waste form. Even for liquid waste streams with ample water, the water chemically bound in the system can add significantly to the total weight; water comprises over 40% of the molecular weight of magnesium potassium phosphate hexahydrate, for example. The additional weight and volume can increase the already significant costs of storing and disposing of radioactive wastes.